CCIE Chapter 6: IP routing Resources: Cisco Press CCNP Self-Study BCMSN Official Exam Certification Guide 4th Edition CCIE_Professional_Development_Routing_TCP-IP_Volume_I How a packet is routed 1. A router receives the frame and checks the received frame check sequence (FCS); if errors occurred, the frame is discarded. The router makes no attempt to recover the lost packet. 2. If no errors occurred, the router checks the Ethernet Type field for the packet type, and extracts the packet. The Data Link header and trailer can now be discarded. 3. Assuming an IP packet, the router checks its IP routing table for the most specific prefix match of the packet’s destination IP address. 4. The matched routing table entry includes the outgoing interface and next-hop router; this information points the router to the adjacency information needed to build a new Data Link frame. 5. Before creating a new frame, the router updates the IP header TTL field, requiring a recomputation of the IP header checksum. 6. The router encapsulates the IP packet in a new Data Link header (including the destination address) and trailer (including a new FCS) to create a new frame. Fast switching, also know as route once switch many, this works by following the steps above for the first packet of a particular flow. The information like destination Mac address etc that needs to be changed is added to a cache. The next time the router sees a packet matching the same flow it doesn’t need to recalculate all the information it can get most of it form the cache. This reduces the amount of processor load need to route the packet. CEF: cisco express forward, CE using a thing called FIB ( forward information base) the FIB contains information about all the known routes in the routing table. The FIB is made up of a tree like structure called mtrie, in the FIB table the most specific routes are entered first. This is done because CEF will match on the first entry it finds in the FIB table so if there where two routes 192.168.1.128/25 and 192.168.1.0/24 unless the /25 entry was first in the FIB table the less specific route would be chosen. The FIB also contains the next hop address for the route. . For every entry in the FIB table there is a point to an entry In the CEF adjacency table. Packets that cant be routed via CEf are marked as cefpunt and sent to the layer 3 engine, some reasons a packet might be sent to the layer 3 engine: An entry cannot be located in the FIB The FIB table is full The IP Time To Live (TTL) has expired The maximum transmission unit (MTU) is exceeded, and the packet must be fragmented An Internet Control Message Protocol (ICMP) redirect is involved The encapsulation type is not supported Packets are tunneled, requiring a compression or encryption operation An access list with the log option is triggered A Network Address Translation (NAT) operation must be performed (except on the Catalyst 6500 Supervisor 720, which can handle NAT in hardware) CEF hardware optimisations ■ Accelerated CEF (aCEF)—CEF is distributed across multiple Layer 3 forwarding engines, typically located on Catalyst 6500 line cards. These engines do not have the capability to store and use the entire FIB, so only a portion of the FIB is downloaded to them at any time. This functions as an FIB “cache,” containing entries that are likely to be used again. If FIB entries are not found in the cache, requests are sent to the Layer 3 engine for more FIB information. The net result is that CEF is accelerated on the line cards, but not necessarily at a sustained wire-speed rate. ■ Distributed CEF (dCEF)—CEF can be distributed completely among multiple Layer 3 forwarding engines for even greater performance. Because the FIB is self-contained for complete Layer 3 forwarding, it can be replicated across any number of independent Layer 3 forwarding engines. The Catalyst 6500 has line cards that support dCEF, each with its own FIB table and forwarding engine. A central Layer 3 engine (the MSFC3, for example) maintains the routing table and generates the FIB, which is then dynamically downloaded in full to each of the line cards. Adjacency Table: the adjacency table acts just like a mac address table but each entry is mapped to a particular entry in the FIB. If there is no mac entry in the adjacency table for a FIB entry the entry is maked as “CEFglean” and the first packet matching that entry is sent to the layer 3 engine so it can perform the ARP request. Once the next hop has been found the layer 2 headers and the TTL field needs to be updated. The switch has an additional functional block that performs a packet rewrite in real time. The packet rewrite engine makes the following changes to the packet just before forwarding: ■ Layer 2 destination address—Changed to the next-hop device’s MAC address ■ Layer 2 source address—Changed to the outbound Layer 3 switch interface’s MAC address ■ Layer 3 IP Time To Live (TTL)—Decremented by one because one router hop has just occurred ■ Layer 3 IP checksum—Recalculated to include changes to the IP header ■ Layer 2 frame checksum—Recalculated to include changes to the Layer 2 and Layer 3 headers Classless and Classful routing Classless routing—When a default route exists, and no specific match is made when comparing the destination of the packet and the routing table, the default route is used. Classful routing—When a default route exists, and the class A, B, or C network for the destination IP address does not exist at all in the routing table, the default route is used. If any part of that classful network exists in the routing table, but the packet does not match any of the existing subnets of that classful network, the router does not use the default route and thus discards the packet. Multilayer Switching Mls uses layer 3 information to make a switching decision ( eg CEF), MLS differs slightly because it uses vlan (SVI) interface, routed interfaces and port channel’s. When using VLAN interfaces, the switch must take one noticeable but simple additional step when routing a packet. Like typical routers, MLS makes a routing decision to forward a packet. As with routers, the routes in an MLS routing table entry list an outgoing interface (a VLAN interface in this case), as well as a next-hop layer 3 address. The adjacency information (for example, the IP ARP table or the CEF adjacency table) lists the VLAN number and the next-hop device’s MAC address to which the packet should be forwarded—again, typical of normal router operation. At this point, a true router would know everything it needs to know to forward the packet. An MLS switch, however, then also needs to use Layer 2 logic to decide out which physical interface to physically forward the packet. The switch will simply find the next-hop device’s MAC address in the CAM and forward the frame to that address based on the CAM. Routed port, a routed port is a port that has had the NO switchport command issued on an MLS switch. It acts like a router port on a router does and requires the ip address information to be entered on the port. Etherchannel routed port, can be config’d as above ( no switchport command on channel group and physical port) but the load balancing method should be changed from mac ( either soruce or dest) to ip ( either source or dest) Policy based routing, Configured on an interface and used to make routing decisions that differ from the routing table, can use lots of different metric or matches to make a decision. The decision can be be changing the next hop address or changing something with a header of the packet. Policy routes are noting more then a sophisticated static route. Policy routing can also be used for route tagging, tag the route with an identifier that can be used to distinguish it from other routes.
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