Routing Summarization is a major factor in the success of designing your network. To ensure that your network can scale properly, route summarization is the biggest factor against which to measure your success. Without summarization, you have a flat address design with specific route information for every subnet being transmitted across the network—a bad thing in large networks.
The six time-proven steps to designing a network are as follows:
OSPF external routes are automatically blocked from being redistributed in BGP by default.
A solution to minimize Internet route instability is using Aggregation. Fluctuation of any single route in an Aggregation does not cause fluctuation in the Aggregate itself.
Backdoor routes offer an alternative IGP path instead of external BGP path. Using Backdoor for specific routes, cause the administrative distance to be equal to BGP Local (200), so the IGP with the lower AD will be preferred.
By default, MED is not compared when routes are learned from different ASs. This behavior could be changed using the bgp always-compare-med command. When the bgp deterministic-med command is enabled, routes from the same autonomous system are grouped together, and the best entries of each group are compared. (useful link) An example of BGP table looks like this:
entry1: AS(PATH) 100, med 200, external, rid 1.1.1.1
entry2: AS(PATH) 500, med 100, internal, rid 172.16.8.4
entry3: AS(PATH) 500, med 150, external, rid 172.16.13.1
+ bgp deterministic-med Enabled, bgp always-compare-med Disabled: There is a group for AS 100 and a group for AS 500. The best entries for each group are compared. Entry1 is the best of its group because it is the only route from AS 100. Entry2 is the best for AS 500 because it has the lowest MED. Next, entry1 is compared to entry2. Since the two entries are not from the same neighbor autonomous system, the MED is not considered in the comparison. The external BGP route wins over the internal BGP route, making entry1 the best route.
When passing an EBGP route to an IBGP neighbor, the EBGP neighbor is set as the next-hop.
In NMBA partial-mesh networks, sometimes the next-hop-self command is required.
When connecting to an Internet eBGP neighbor, AS_PATH list that contains Private ASs, should be stripped.
An AS that advertises an Aggregate, considers itself the originator of that route, irrespective of where that route came from. This issue can cause a loop, so the solution is to use the AS_SET parameter before an aggregate-address.
Sometimes, customers prepend fake ASes, to prevent becoming a transit AS for providers.
Combine route injection in BGP with static Routes (with distance 254, for example) to Null0 if you want to prevent route fluctuation even if your IGP routing is not stable.
In most of the situations, METRIC is used for inbound traffic management and LOCAL_PREFERENCE is used for outbound traffic administration.
BGP multipath can be used to install multiple paths in the IP routing table if the paths are learned via the same neighboring AS. The maximum-paths command can be used to install up to six paths to a single destination. The following attributes of parallel paths have to match with the best path:
Service Provider should filter some IP prefixes in incoming updates, such as RFC1918. Because a customer should only advertise its global networks to the Service Provider.
Multihomed Customers should avoid becoming a Transit-AS. As by default in most of the cases the tie breaker for BGP is the Shortest AS-Path, so the providers connected to the to customer will use the customer link as a Transit-AS to reach each other.
Service Providers should filter Private addresses in incoming updates of Customers.
In a scenario where a customer has two border routers without IBGP, and IGP inside the AS, there will be no loops, but if running IBGP between the border routers, special care should be taken or a direct link between the two border routers is required.
Policy Routing only affects the Next-Hop. The destination is unchanged!
Policy Routing is CPU intensive, because it is based on the source unlike Dynamic and static routing. So, when routing based on the destination there is no need of Policy Routing.
Customers can only affect their outgoing traffic, and can’t directly affect incoming traffic.
(config)# ip as-path access-list# [permit/deny] regexp
(config-router)# neighborip-addressfilter-listas-path-filter# [in/out]
The role of TE is to get the traffic from edge to edge in the network in the most optimal way.
MPLS TE takes into account the configured (static) bandwidth of links.
MPLS TE takes link attributes into account (for instance, delay, jitter).
MPLS TE adapts automatically to changing bandwidth and link attributes.
MPLS TE allows for a TE scheme where the head end router of a label switched path (LSP) can calculate the most efficient route through the network toward the tail end router of the LSP. The head end router can do that if it has the topology of the network. Furthermore, the head end router needs to know the remaining bandwidth on all the links of the network. Finally, you need to enable MPLS on the routers so that you can establish LSPs end to end. The fact that label switching is used and not IP forwarding allows for source-based routing instead of IP destination-based routing. That is because MPLS does forwarding in the data plane by matching an incoming label in the label forwarding information base (LFIB) and swapping it with an outgoing label.
Therefore, it is the head end label switching router (LSR) of the LSP that can determine the routing of the labeled packet, after all LSRs agree which labels to use for which LSP.
Process switching is the slowest of all switching methods. When switching a packet through the router, a Cisco IOS process copies the packet to the CPU memory and looks up the destination IP address in the IP routing table. Based on the outcome of this lookup, the process switches out the packet on a particular interface after it does some housekeeping on the IP header.
Fast Switching
The first packet for a destination that arrives is process switched. The switching of the first packet by the central CPU gives the CPU the opportunity to build a cache. This cache is called the IP fast switching route cache and is used by the interrupt code to switch subsequent packets for the same destination.
If a prefix changes in the routing table, the entry in the fast switching cache is invalidated, and the first packet for a destination has to be process-switched again to build the new entry in the route cache.
Each IP prefix entry in the route cache has an outgoing interface, next hop, and Layer 2 rewrite field. This Layer 2 rewrite (or MAC rewrite) is the information that the router needs to change in the Layer 2 frame header when the frame is rebuilt so that it can be sent on the outgoing interface. Continue reading “MPLS Fundamentals: 4 – CEF”
