Routing Mnemonics

I’ve been keeping track of some the mnemonics that I have come across or have figured out for myself. For instance in general, in layer 2 elections the lower priority usually wins, however, in layer 3 elections the higher priority usually wins.

Layer 2

LACP System Priority
2-bytes priority values followed by a 6-byte MAC address. Lowest system priority makes decisions about the etherchannel setup.

LACP Port Priority
LACP port priority is a 2-byte priority followed by a 2-byte port number. Lowest port priority is used to decide which ports are put in standby mode when not all ports can be put in etherchannel.

Root bridge election, lowest bridge ID wins. Bridge ID consists of:
–2-byte bridge priority from 0-65,535 with a default of 32,768.
–6-byte MAC address
If the bridge priorities are equal, lowest MAC wins.

Root port, lowest root path cost.

Designated port — lowest root path cost or if equal use ties breakers:
1. Lowest root bridge ID
2. Lowest root path cost to root bridge
3. Lowest sender bridge ID
4. Lowest sender port ID

Frame Relay
DCE requires the clock rate, DCE, DTE, clock rate starts with a c and DCE is the one with a c in it.

Layer 3

Active router election is based upon priority, highest priority wins. Default priority of 100 and a range of 0-255. Highest IP address on HSRP interface breaks ties.
Standby router is the second highest priority.

Election of master is the router with the gateway IP address or if not a “real” IP address, the router with the highest priority. Priority ranges from 1 to 254 with 254 being highest, 100 is the default.

Active virtual gateway (AVG) is elected by the highest priority value, tie breaker is the highest IP address in the group. Router priority is 1-255 with 255 being highest, 100 is the default.

OSPF DR/BDR Election
1. Highest priority wins.
2. Highest router ID breaks ties.
Priority range is 0-255 with 255 being highest, 1 is the default and 0 means the router will not participate in the election.

1. router-id command wins.
2. If no router-id is set, the highest loopback address wins, even if it is not advertised and it is not advertised by default.
3. Highest physical address wins.

OSPF summary-address command or the range command.
The summary-address command is used on an ASBR and has an “S” in it, whereas the area range command is used on an ABR and does not have an “S” in it. Both commands are used to summarize routes.

OSPF ExStart
During ExStart of the OSPF packet exchange the neighbor with the highest RID will become the master and sets the DD sequence number.

An exception to the rule of Layer 2 lower takes priority and Layer 3 higher takes the priority. If two routers are the same distance from the source, the router with the numerically lower IP address becomes the designated forwarder for the network.

BGP best path mnemonic
We love oranges as oranges mean pure refreshment.

We — Weight (highest)
Love — LOCAL_PREF (highest)
Oranges — Originate (local)
AS — AS_PATH (shortest)
Oranges — Origin Code (IGP > EGP > Incomplete)
Mean — Med (lowest)
Pure — Paths (External > Internal)
Refreshment — RID (lowest)

RIP and any other protocol that has the letters R-I-P in it requires a seed metric, RIP, IGRP, EIGRP.

This entry was posted in Routing. Bookmark the permalink.

Leave a Reply

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s