Route Summarization and Variable Length Subnet Masks (VLSM)

Route Summarization and Variable Length Subnet Masks (VLSM)

Route Summarization reduces the size of routing tables while maintaining routes to all destinations in the network. Also improves convergence time (no longer has to announce changes).

VLSM means that more than one subnet mask value is used.

* Many networks use mask 255.255.255.252 for serial point-to-point links.

* On LAN subnets, mask of 255.255.255.0 could be used as well.

* VLSM is required in order to summarize routes

* Without summarization, 4 routes are recorded for Yosemite, 4 for Seville.

* With summarization, only 1 route for each subnet is used.

Configuring Route Summarization

(perform on each router)

configure terminal

interface serial0/0

ip summary-address eigrp 1 [address][subnet mask]

* Routers now only advertise the "summarized" route to other routers

* show ip route will say "Variably subnetted" to indicate multiple masks are used.

* Summarized routing table includes network id with outgoing interface set to null0, meaning a packet can be sent to correct network, but no matching subnet exists, packet is discarded.

VLSM Requirements: subnets do not overlap, and the routing protocol supports it.

Interior IP Routing Protocol VLSM Support

Routing Protocol	Support VLSM
RIPv1	No
IGRP	No
RIPv2	Yes
EIGRP	Yes
OSPF	Yes

Finding the "Best" Summary Subnet Mask

1. Write down all subnets in binary form.

2. Find where the first difference occurs.

3. Use that subnet mask.

Example:

10.2.1.0 = 0000 1010 0000 0010 0000 0001 0000 0000

10.2.2.0 = 0000 1010 0000 0010 0000 0010 0000 0000

10.2.3.0 = 0000 1010 0000 0010 0000 0011 0000 0000

10.2.4.0 = 0000 1010 0000 0010 0000 0100 0000 0000

Set the values that aren’t bold to 1, set all other values to 0, and use that as your mask:

255.255.248.0

Classless and Classful Routing Protocols

Classful don’t transmit the subnet mask information in updates.

Classless do transmit the subnet mask information in updates.

Autosummarization: In classful routing, uses Static Length Mask (uses the traditional Class A, B, and C masks).

• RIP and IGRP perform this by default (can’t disable)
• RIPv2 and EIGRP, can either be enabled or disabled

Contiguous Network: Single Class A, B, or C network for which all routes to subnets of that network pass through only other subnets of that same single network

Discontiguous Network: Only routes to one subnet pass through route of another subnet of a different network.

Analogy Lower 48 states are contiguous because you can drive to any point without needing to go through another country (subnet/network). To get from Alaska to one of the lower 48 states, you must go through Canada (a separate network), so it is discontiguous.

** Classful protocols do not support discontiguous networks. **

To migrate from classful to classless

Configure using classless protocol, disable autosummarization.

l and Classless Routing

Classful & Classless Routing Default Routes are best when only a single path exists to a part of the network.

Also called Gateway of Last Resort

Configure:

ip route 0.0.0.0 0.0.0.0 [IP address of next hop]

-or-

: ip default-network [address]

In show ip route command, * (asterisk) denotes it is a candidate for default route.

'Toggle between classful/classless: 'ip classless and no ip classless

Classful Logic

Match network in routing table

Look up specific subnet

If exists, forward, otherwise, discard

Classless Logic

Lookup specific subnet

If exists, forward, otherwise, send to default gateway