What is icmp redirect

Due to this fact and the security risks involved in such scenario, it is still a recommended practice to disable ICMP redirect messages ignore them from all public interfaces. Mind that if forwarding is disabled we are not a router value of net. Ubuntu Community Ask! Sign up to join this community. The best answers are voted up and rise to the top.

Stack Overflow for Teams — Collaborate and share knowledge with a private group. Create a free Team What is Teams? Learn more. What are ICMP redirects and should they be blocked? Ask Question.

Asked 9 years, 6 months ago. Active 4 years, 2 months ago. Viewed 79k times. Improve this question. Add a comment. Active Oldest Votes. Some network environments, however, require that these settings are disabled so review and enable them as needed. Improve this answer. Community Bot 1. Manish Sinha Manish Sinha You have to do this to accept the changes: sudo sysctl -p — user I don't think setting net. Full fix would be then: net. Marek Marek 31 1 1 bronze badge. However, for the purposes of this example assume no such configuration is present.

Assume Router A loses connectivity to Network X as shown in the picture. When packets from the customer Network Y or remote Network Z try to reach Network X, Routers A and B will bounce the traffic between each other, decrementing IP Time-To-Live field in every packet until its value reaches 1, at which point further routing of the packet is not possible.

To do this, CPU on Nexus Supervisor module needs to obtain IP address information of the flow whose path through the network segment can be optimized. This is the reason behind data packet being sent by ingress Linecard to the Supervisor module. At the Linecard level the process starts in the form of hardware forwarding exception.

Exceptions are raised on ASICs when packet forwarding operation cannot be successfully completed by the Linecard module.

In this case, data packet needs to be sent to the Supervisor module for correct packet handling. While Nexus Supervisor modules use powerful CPU processors that are capable of processing large volumes of traffic, the platform is designed to handle most of the data traffic at the Linecard level without engaging Supervisor's CPU processor in packet forwarding process. This allows CPU to focus on its core tasks, leaving packet forwarding operation to dedicated hardware engines on Linecards.

In stable networks, packet forwarding exceptions, should they occur, are expected to happen at a reasonably low rates.

With this assumption, they can be handled by Supervisor CPU without significant impact on its performance. On the other hand, having CPU deal with packet forwarding exceptions that occur at a very high rate can have a negative effect on overall system stability and responsivness.

Nexus platform design provides a number of mechanisms to protect switch CPU from being overwhelmed by significant amount of traffic. These mechanisms are implemented at different points in the system. Both set traffic rate thresholds, effectively controling amount of traffic to be forwarded to the Supervisor from each Linecard module.

These protective mechanisms give preference to the traffic of various control protocols that are critical for network stability and switch manageability, such as OSPF, BGP or SSH, while aggressively filtering types of traffic that are not critical to control plane functionality of the switch. Most of the data traffic, if forwarded to the CPU as a result of packet forwarding exceptions, is heavily policed by such mechanisms. While hardware rate limiters and CoPP policing mechanisms provide stability of control plane of the switch and are strongly recommended to be always enabled, they can be one of the main reasons of data packet drops, transfer delays, and overall poor application performance across the network.

This is done with show ip traffic command. Picture 7 shows scenario similar to the one on Picture 3. Here Network X is replaced by Host Timestamps in above output suggest that three packets highlighted in this example were captured at the same time, Below is a per-packet breakdown of this packet group.

While navigating through large Ethanalyzer captures that have many packets of different types and flows, it may not be easy to correlate ICMP Redirect messages with corresponding data traffic. In these situations, focus on ICMP Redirect messages to retrieve information about sub-optimally forwarded traffic flows. ICMP Redirect messages include the internet header plus the first 64 bits of the original datagram's data. This data is used by the source of the datagram to match the message to the appropriate process.

Use Ethanalyzer packet capture tool with detail keyword to display content of ICMP Redirect messages and find IP address information of the data flow which is sub-optimally forwarded. If network design requires traffic flow to be routed out of the same Layer 3 interface on which it entered the switch or router, it is possible to prevent the flow from being routed through the CPU by explicitly disabling ICMP Redirect functionality on corresponding Layer 3 interface.

In the early days of the Internet such optimisation helped to save expensive network resources, like link bandwidth and routers' CPU cycles. As network bandwidth became more affordable, and relatively slow CPU-based packet routing evolved into faster Layer 3 packet forwarding in dedicated hardware ASICs, the importance of optimal data transit through multi-point network segments decreased, and is not getting as much attention of network designers today as it used to.

However, its attempts to notify network nodes on multi-point Ethernet segments about optimal forwarding paths are not always understood and acted upon by network personnel. In networks with combined use of various forwarding mechanisms, such as Static Routing, Dynamic Routing and Policy-Based Routing, leaving ICMP Redirect functionality enabled without proper monitoring may result in undesirable use of transit node s CPU to handle production traffic.

This, in turn, may cause significant impact both on production traffic flows and on control plane stability of network infrastructure. For most networks it is considered a good practice to proactively disable ICMP Redirect functionality on all Layer 3 interfaces in network infrastructure. This helps to prevent scenarios of production data traffic being handled in CPU of Layer 3 switches and routers when there is a better forwarding path through mutli-point network segments.

Skip to content Skip to search Skip to footer. Available Languages. Download Options. Updated: October 17, Contents Introduction. When Host sends a packet to destination network X, the following happens 1.

However, if Host uses ICMP Redirect messages to adjust its routing cache and starts sending subsequent data packets directly to G2, the following benefits are achieved in this scenario optimisation of data forwarding path through the network; traffic reaches its desination faster reduction of network resources utilization, such as bandwidth and router CPU load As shown in Picture 2 , after Host created route cache entry for Network X with G2 as its next hop, these benefits are seen in the network: bandwidth utilization on the link between Switch and router G1 decreases in both directions CPU utilization on router G1 reduces, because traffic flow from Host to Network X does not traverse this node anymore end-to-end network delay between Host and Network X improves.

W hen Host sends packet to destination Network X, the following happens in the network 1. Static Routing To illustrate this, consider scenario in Picture 4. Note : For more information on Ethanalyzer, refer to Ethanalyzer on Nexus Troubleshooting Guide Picture 7 shows scenario similar to the one on Picture 3.

Use the following command to capture ICMP traffic received and sent by Nexus CPU Nexus ethanalyzer local interface inband capture-filter icmp limit-captured-frames Capturing on inband Below is a per-packet breakdown of this packet group First packet is the ingress data packet, which in this example is an ICMP Echo Request. This packet is sent back to the host.

Use Ethanalyzer packet capture tool with detail keyword to display content of ICMP Redirect messages and find IP address information of the data flow which is sub-optimally forwarded Nexus ethanalyzer local interface inband capture-filter icmp limit-captured-frames detail Frame 2 70 bytes on wire, 70 bytes captured Arrival Time: Sep 15, Disable ICMP Redirects If network design requires traffic flow to be routed out of the same Layer 3 interface on which it entered the switch or router, it is possible to prevent the flow from being routed through the CPU by explicitly disabling ICMP Redirect functionality on corresponding Layer 3 interface.

Follow these steps to verify that ICMP Redirect functionality is disabled ensure no ip redirects command is added to interface configuration Nexus show run interface vlan 10 interface Vlan10 no shutdown no ip redirects ip address Contributed by Cisco Engineers Nikolay Kartashev.

Was this Document Helpful? Yes No Feedback.